In order to prepare for the evaluation of vaccines for the prevention of HIV infections this project will enroll individuals from several populations at high risk of HIV infection in northern Thailand. Included will be a cohort of about 1200 recruits into the Royal Thai Army (RTA) and Royal Thai Air Force (RTAF) in Chiang Mai, individuals (n=1000) who present at sexually transmitted disease (STD) clinics run by the Ministry of Public Health (MOPH) in northern Thailand and about 800 recruits in the RTA or RTAF who will be discharged from the military in April 1993 and October 1993. These recruits were initially enrolled in cohort studies to determine the incidence, prevalence and risk factors for HIV infection in April 1991 and November 1991. Criteria for enrollment in the present study of high risk populations will be HIV seronegativity at baseline, age of 16 years or older and agreement to participate in a longitudinal study. At 3-6 month intervals participants will be studied with a detailed behavioral and sociodemographic risk factor questionnaire, clinical evaluation for STDs and repeat serological testing for HIV and syphilis. The goals of this study are to measure the incidence of HIV infections, to identify biological and behavioral co-factors for incident HIV infection, and to determine our success in continuing these high risk populations in follow-up. We will also estimate what proportion of individuals in these populations might participate in a future trial of an HIV vaccine. This estimate will be done by interview and by screening individuals for markers of hepatitis B virus (HBV) infection and offering HBV vaccine to susceptibles. Those who accept and comply with the HBV vaccine will be compared to non-acceptors and non-compliant individuals. We will collaborate with scientists at NIAID in studies of the genetic and antigenic variation of HIV isolates from infected individuals in northern Thailand. Additionally, this project will strengthen the infrastructure, field management and research capabilities of our collaborators at Chiang Mai University, the Ministry of Public Health and the RTA and RTAF in northern Thailand, so they will be better prepared to carry out HIV vaccine trials in this population in the future. Thailand was judged by a team from WHO to be one of four developing countries best suited to conduct HIV vaccine trials. Preparation for these trials by enrolling individuals from high risk populations in longitudinal studies of incident HIV infection, as proposed in this application, is a necessary first step in the evaluation of HIV vaccines.